This invention relates to conservation of hydrogen in hydrogen recycle processes used in oil refineries and petrochemical plants. More specifically, the invention relates to a method, applicable only to hydrogen-consuming hydrogen recycle processes, of reducing the amount of hydrogen vented from the processes. Hydrogen recycle processes can be classified into two types: those which produce hydrogen and those which consume hydrogen. Examples of hydrogen-producing processes are catalytic reforming and the various dehydrogenation processes. Hydrogen-consuming processes include hydrogenation, hydrodealkylation, hydrodesulfurization, hydrocracking, and isomerization. FIGS. 1 and 2, which are presented herein as examples, show the basic flow arrangement of hydrogen-consuming hydrogen recycle processes. A circulating gas flow consisting mainly of hydrogen and including hydrocarbon vapors is maintained in the equipment loop by means of a compressor. A hydrocarbon charge stock stream and a hydrogen stream are added to the loop. A liquid product stream and a vent stream are removed from the loop. It is desirable to maintain the concentration of hydrogen in the reactor above a certain minimum value for each particular process in order to protect catalyst activity and stability and/or product yield structure. These minimum values are known to those skilled in the art by means of experimental data which has been collected by them. If the hydrogen concentration falls below the minimum value in a process where the reactor contains catalyst, the result will be excessive deposit of coke on the catalyst, premature deactivation of the catalyst, and reduction of product yield. In those processes which do not utilize a catalyst, the hydrogen concentration must be maintained above the minimum value in order to protect the yield structure, that is, to maximize the amount of desired product produced by the processing unit and minimize the production of undesirable by-products. A standard method for maintaining the required minimum hydrogen concentration is to control the rate at which gas in circulated by adjusting compressor capacity and to control vent gas flow. The operator of the hydrocarbon processing unit monitors the quantity of circulating gas flowing by means of a flow indicator and manually adjusts compressor capacity. Alternatively, an automatic flow controller can be used to maintain the quantity flowing at an appropriate value above the minimum. An automatic flow control loop is used to control the vent flow at a previously determined value. The vent flow is made necessary by the presence of light hydrocarbons in the circulating gas stream. Some light hydrocarbons enter the system as part of the hydrogen feed stream, which is not pure hydrogen, and some are produced in side reactions taking place in the reactor. While some of the light hydrocarbons leave the system dissolved in the liquid stream, there is usually an increase in concentration over time unless a vent stream is employed. Thus the purpose of the vent stream is to remove light hydrocarbons from the process, as they would interfere with the desired reactions. The principle is similar to that of cooling tower blow-down, where a continuous stream of water is withdrawn to keep water hardness at an acceptably low level. The vent stream usually contains 60 mole percent or more of hydrogen, so there is a significant hydrogen loss from the system. The vented gas is usually routed to the refinery fuel gas system. However, hydrogen has more value in the process than it does as fuel, so that it is economically desirable to reduce the amount vented.
Variations in cooling efficiency lead to an excessive vent gas flow. The cooling medium used in the cooler, which is part of the equipment loop shown in FIGS. 1 and 2, is water or ambient air. The temperature of the cooling medium varies with weather conditions and time of day and can vary from hour to hour. As the cooling medium temperature falls, a larger quantity of hydrocarbon vapor condenses out of the cooled stream, thus causing the concentration of hydrogen in the circulating stream to increase. The average molecular weight of the circulating gas decreases as hydrogen concentration increases. The flow meter used is normally of the orifice type. As can be seen from an inspection of the well-known orifice flow meter equation and the example presented herein, a lower molecular weight of the flowing gas results in a lower flow reading. This lower flow reading causes the automatic controller to increase vent gas flow in order to bring the flow reading back up to its proper value. However, the flow reading is not an accurate indication of hydrogen concentration, because of the changed molecular weight, and the effect is an increase in vent gas flow which is not necessary to protect the catalyst and does not serve any other desired purpose. Thus a decrease in the temperature of a cooling medium which is capable of varying from hour to hour results in an unnecessarily large vent gas flow. Variations in cooling efficiency lead to an excessive loss of hydrogen by means of the false decrease in vent flow indication due to lower average molecular weight. It is possible to control circulating gas composition at a constant value by regulating the quantity of cooling medium passed through the cooler. However, this is not usually a desirable option, since a colder cooling medium yields a colder gas-liquid separator, which enhances liquid product recovery and the purity of the circulating gas. Also, a colder gas-liquid separator results in less hydrogen being dissolved in the liquid product stream and therefore lost from the system.
The art which has been discovered which is closest to the instant invention is disclosed by Bajek and McLaughlin in U.S. Pat. Nos. 3,974,064 and 3,972,804. These patents present a comprehensive control scheme for hydrogen recycle processes. The instant invention can be considered an improvement on those inventions. Those effects of changes in cooling medium temperature which are adverse are recognized and control action is taken to mitigate them. The key process parameter of partial pressure is recognized and used to initiate control action. An excessively large safety margin in hydrogen concentration is not necessary.